A number of prior proposals have been made for filling a honeycomb panel with tubular cells with a reinforcing foam so as to provide an enhanced compression strength which is commonly significantly greater than the compression strength of the panel and the foam separately.
U.S. Pat. No. 5,338,594 (Wang) issued Aug. 6, 1994 assigned to Hexcel Corporation discloses a method for filling the cells where a foamable precursor film is applied to one face of the panel and held between two plates and placed in a hot press and heated under pressure. A release cloth is applied over the ends of the panel which is porous to allow the escape of the gases as the foaming action occurs. However this arrangement requires heat and pressure and hence is inconvenient. There is also a risk that the foam does not properly fill the cells with a constant density foam without voids.
U.S. Pat. No. 6,156,146 (Cundiff) issued Dec. 5, 2000 assigned to Boeing discloses using such a foam filled honeycomb product in a Resin Transfer Molding process.
U.S. Pat. No. 6,387,200 (Ashmead) issued May 14, 2002 assigned to Cellbond composites Limited discloses a method of filling the cells where a fibrous mat is placed over each face of the panel before the foam is injected through the mat into the cells so that the foam is absorbed into the mat to form a fiber reinforces skin at the end face of the honeycomb panel. Again this method has significant difficulty in properly filling the cells to the required constant density without voids.